Workpieces of the type referred to are generally sintered in an oven under a protective atmosphere preventing their oxidation, specifically a substantially oxygen-free gas which usually contains combustible constituents such as carbon monoxide. When the particles are permeated by a lubricant, e.g. an organic substance such as zinc stearate or a wax, the sintering oven may comprise a muffle furnace in cascade with a preheating chamber which the workpieces must traverse before sintering and wherein the lubricant is volatilized. The protective gas, passing in counterflow to the advancing workpieces through the preheating chamber, is designed to sweep out the vaporized lubricant and to prevent its recondensation on the oncoming workpieces entering the chamber in a relatively cold state, normally at room temperature. It is difficult, however, to control the chamber temperature precisely enough to avoid a thermal decomposition of the lubricant with resulting precipitation of elemental carbon on the workpieces. The carbon in that case would form a hard coating so as to seal the interstices of the particulate bodies and prevent the evaporation of remaining lubricant whose gas pressure could then destroy the workpieces.
Attemps have been made to deal with this problem by a very slow heating of the workpieces. This, of course, has the drawback of requiring rather lengthy preheating chambers and extended treatment periods.
Another known solution involves the admission of an additional flow of hot low-oxygen gas into the preheating chamber. This flow is generated by the combustion of gaseous hydrocarbons (e.g. natural gas) with limited air supply in a burner attached to the exit end of the preheating chamber. The resulting increase in the quantity of protective gas and of its flow rate in that chamber reduces the transit and heating time of the workpieces, in comparison with the aforedescribed procedure, but has the drawback of practically doubling the amount of combustible components at the chamber entrance. These partly poisonous components burn upon leaving the chamber and mixing with the ambient air. This leads to an undesirable rise in the temperature around the sintering oven and is inconvenient for the operating personnel. A further disadvantage is the energy loss involved in the preheating of the additional gas mass and the expense of installing and operating the burner.